This invention relates generally to television receivers, and is particularly directed to a system for initiating operation of a television receiver.
In a conventional television receiver rectified AC line voltage is filtered to provide line referenced and line isolated D.C. B+ operating voltages. The line reference B+ voltage is used to, among other things, develop the electron accelerating potential in the television receiver's cathode ray tube, regulate electron beam intensity, and drive horizontal and vertical electron beam deflection circuitry. The line isolated B+ voltage is used to energize the television receiver's video, chrominance and tuner circuits as well as to provide a voltage supply for horizontal and vertical deflection control circuitry.
Included in the horizontal deflection control circuitry is a horizontal oscillator which provides switching signals to an electron beam trace control circuit of the horizontal deflection system for developing scanning current in a horizontal deflection winding. After the elapse of a start-up interval, rectified retrace pulses provide an energizing line isolated B+ voltage to the horizontal oscillator. A secondary winding of a horizontal output transformer is generally used to derive the line isolated B+ voltage after operation of the horizontal deflection circuit has been initiated. However, until the horizontal oscillator circuit begins oscillation, a line isolated voltage alternating current is not available from the flyback transformer. Thus an initial pulse of current is necessary to start the oscillator circuit which then causes the horizontal output circuit to generate the alternating current in an extra winding in the flyback transformer.
One approach to providing the initial pulse of current for initiating horizontal oscillator operation involves the use of a start-up transformer energized by the common power supply of the television receiver for delivering a start-up pulse to the horizontal oscillator for initiating the operation thereof. When the television receiver is turned on, an initial period occurs in which inrush current flow into an uncharged filter capacitor. The start-up circuit includes a winding coupled magnetically to an inductor in the path of the inruch current. This winding develops an alternating current potential during this initial period, which is rectified to provide a line referenced B+ start-up operating voltage for various television receiver systems including the horizontal deflection circuit. One example of this type of television start-up circuit is described in U.S. Pat. No. 4,127,875 to Fernsler et al. U.S. Pat. No. 4,112,465 to Willis describes an improvement in the aforementioned approach involving the decoupling of the alternating current potential developed in the start-up transformer from the television receiver circuits for the entirety of the steady state interval in order to eliminate unwanted modulation of the raster when the start-up alternating potential is provided to the horizontal deflection circuit after initial television receiver turn-on. Other examples of a start-up power supply for a television receiver utilizing a start-up transformer or winding can be found in U.S. Pat. Nos. 3,947,632 to Giger et al. and 4,188,641 to Baker et al.
Another approach to providing an initial pulse of current to start the television receiver oscillator circuit in initiating the operation thereof involves the use of a capacitor voltage divider network, sometimes referred to as a "kick" circuit for generating a surge voltage from the common power supply of the television receiver. This surge voltage is applied to the horizontal oscillator to cause it to begin oscillation and thereafter the rectified voltage derived from the television receiver's flyback transformer provides the necessary operating voltage for continued horizontal oscillator operation. Examples of television receiver start-up systems employing a condenser voltage divider network are provided in U.S. Pat. No. 3,814,851 to Nakagawa et al. and No. 3,621,134 to Waring.
U.S. Pat. No. 4,246,634 to Purol discloses a switched mode power supply start-up circuit in which an oscillating switching circuit is connected in series with a transformer and a direct current source so as to chop the direct current source in energizing the transformer. Power for starting the oscillator, which requires a substantially lower voltage than that of the rectified source, is derived from a capacitor charged by the direct current source. U.S. Pat. No. 4,240,013 to Wedam discloses a start-up arrangement including a transistorized push-pull output switching stage comprised primarily of an astable multivibrator which is made self-oscillatory upon engagement of the ON switch. After the initial start-up interval, when a sufficient voltage has been developed to permit sustained horizontal oscillator and driver operation, the push-pull output switching stage ceases to self-oscillate with the horizontal oscillator then energized by a low voltage DC power supply in a conventional manner.
All of the systems described above require components dedicated solely to power supply start-up and which cease to function following stable power supply operation. Such dedicated components performing a specialized, limited function increase the cost and complexity of the power supply. The present invention, however, provides a power supply start-up system particularly adapted for use in a conventional television receiver which employs a minimum number of components limited to the specific task of power supply, or television receiver, start-up.